Solder glasses find a number of applications, such as in sealing capacitors (U.S. Pat. No. 3,770,404); joining components of colored T.V. picture tubes (U.S. Pat. No. 3,770,404) and as "tubesheets" in hollow fiber-type sodium/sulfur battery cells.
The latter application requires the use of very finely ground solder glasses, which cannot be produced by ordinary grinding methods.
U.S. Pat. Nos. 3,476,602, 3,765,944 and 3,791,868 provide detailed descriptions of high temperature, sodium/sulfur cells, wherein the electrolyte/separator takes the form of a large number of Na.sup.+ ion-conductive, glass, capillary tubules or hollow fibers. Briefly, however, the cells may be described as comprising a generally cylindrical container for the catholyte (Na.sub.x S.sub.y, for example) and a generally cylindrical reservoir for the anolyte (Na.degree., for example) which are abutted against and joined in sealing arrangement to an intervening, horizontal, electronically non-conducting "tubesheet" disc. Fine, hollow glass fiber lengths having their lower ends closed and their upper ends open pass through the tubesheet in sealing engagement therewith. The open ends of the fibers communicate with the molten alkali metal in the anolyte reservoir and the portions of the fibers dependent from the tubesheet are immersed in the molten catholyte. U.S. Pat. No. 3,791,868 describes a method of forming such a cell in which the glass fiber lengths are arrayed in parallel on a metal foil strip and a band of a tacky paste comprising a powdered solder glass is deposited on and between the fibers adjacent their open ends. The foil and fibers are then rolled into a bundle, the paste is devolatilized and the glass particles joined to form the tubesheet. The foil is employed as the cathodic current collector in the cell and the anolyte reservoir itself functions as the anodic current distributor.
The glass component of the paste preferably is a high borate glass.
U.S. Pat. No. 3,917,490 discloses that it is possible to reduce high borate glass particles to powders suitable for tubesheet fabrication by ball milling them in the presence of at least 0.5 wt. % of an aliphatic amine, for a total of about 7 days or more. Elevated temperatures are not required for the first stage of grinding but are necessary during the second stage--the last 1-3 days of (finish) grinding.
Prior to development of the preceding method, no way of fine-grinding high borate glasses was known. However, a substantial reduction in the grinding time required for this method would constitute a considerable further advance.
Application of the patented method to production of glass fines for utilization in tubesheets could also stand improvement in another respect. That is, it has been considered that really efficient grinding of the -325 mesh solder glass particles in the feed to the amine grind cannot be achieved if that feed also includes +325 mesh particles. This necessitates a laborious and time-consuming sieving operation. Any substantial reduction in sieving requirements would be highly desirable.
The '490 patent teaches that any other suitable type of mill can be employed in place of the porcelain ball mill actually used in the examples. That is, the glass particles can be ground with the amine in a porcelain mill containing porcelain or metal balls or in other mills which are the functional equivalent thereof. There is, however, no suggestion in the patent that any improvements might result from using other types of mills.
Furthermore, the substitution of a rod mill for the ball mill is generally contraindicated when the product powder is to be used to fabricate articles in which close packing of the particles is essential. That is, the particle size distribution known to be characteristic of ball-ground materials is more suitable for attainment of close packing (which is a prerequisite for impermeable tubesheets, for example). The particle size distribution of materials ground by rod milling is generally considered to be less amenable to close packing.
In order to be suitable for fabrication of tubesheets by presently known methods, the (amine-coated) glass powder produced in the grinding operation must be capable of forming a thixotropic, high-solids content paste or slurry with a volatilizeable suspending agent, such as (for example) cumene containing 1-2 wt. % of 1-hexadecylamine. That is, the paste must be extrudeable under pressure but also capable of maintaining the shape imposed on it until it is devolatilized, etc.
The slurries actually used in making such tubesheets not only include the glass powder but also about an equal weight of highly spherical particles, of the same glass, having diameters within the range of from about 45 to about 104 microns. This combination of larger, essentially spherical particles, and 25.mu. or less, more irregularly shaped particles having the size distribution patterns characteristic of ball milled fines approximate, in performance, a mixture of somewhat smaller (.about.80.mu.) spheres and spherical fines, corresponding to the "binodal" size distribution which would be considered ideal for close packing between 70-80.mu. diameter fibers spaced about 200-400.mu. apart.
It has been found that in order to yield finished fines which--when used as above-described--yield strong, impermeable tubesheets, the product of the first stage of grinding should meet certain specifications.
That is, it is generally desirable that the particles to be finish-ground have volume average diameters greater than 7, up to at least 11 microns and at least 95 wt. percent of the particles have maximum dimensions less than 25.mu.; also, the dispersity factor (ratio of volume average diameter to number average diameter, or .SIGMA.(nD.sup.4)/.SIGMA.(nD.sup.3).div..SIGMA.(nD)/.SIGMA.n--where n=# of particles having a diameter D) should have a value greater than 4, up to at least 5. Otherwise, the final product is more likely to include too many particles in the 4-8.mu. and 20.mu. and up size ranges and too few in the 10-14.mu. range.
The foregoing specifications can be met by first-stage products made by the method disclosed in the '490 patent, but only at the expense of prolonged grinding times--even if no +325 mesh particles are present in the feed glass. A need for a more efficient first-stage grinding method has been evident for some time, but attempts to modify the disclosed method, such as by using steel balls instead of porcelain "balls" (short cylinders), have not been successful.